132 NATURAL THEOLOGY. 



CHAPTER IX. 



OF THE MUSCLES. 



Muscles, with their tendons, are the instru- 

 ments by which animal motion is performed. It 

 will be our business to point out instances in which 

 and properties with respect to which, the disposi- 

 tion of these muscles is as strictly mechanical as 

 that of the wires and strings of a puppet. 



I. We may observe, what I believe is universal, 

 an exact relation between the joint and the muscles 

 which move it. Whatever motion the joint, by its 

 mechanical construction, is capable of performing, 

 that motion the annexed muscles, by their position, 

 are capable of producing. For example, if there 

 be, as at the knee and elbow, a hinge-joint, capa- 

 ble of motion only in the same plane, the leaders, 

 as they are called, {, e., the muscular tendons, are 

 placed in directions parallel to the bone, so as, by 

 thecontractionor relaxation of the muscles to which 

 they belong, to produce that motion and no other. 

 If these joints were capable of a freer motion, there 

 are n^ muscles to produce it. Whereas, at the 

 shouldi^r and the hip, where the ball and socket- 

 joint allows by its construction of a rotatory or 

 sweeping motion, tendons are placed in such a posi- 



